DESCRIPTION (Applicant's Description): Muscle development is one of the first systems in which genes essential for determination and differentiation were identified. An area of myogenesis that remains largely unresolved, however, is the mechanism by which myoblasts fuse to form multinucleate muscle fibers. While this process is often considered a late aspect of myogenesis, the fact that muscle fibers attain precise sizes in accordance with their locations suggests a highly regulated mechanism. The focus of our research is the process of myoblast fusion in the Drosophila embryo, in which one can utilize the advantages of a combined genetic and molecular approach. A major hypothesis of our research is that similar developmental decisions in different organisms rely on conserved genes and pathways. Consequently, insights provided by a genetically tractable organism may be applicable to higher eukaryotes, in which the identification of genes essential for myoblast fusion has been more problematic. The goals of the proposed research include the characterization of two genes (sns and mbc) that are essential for myoblast fusion. Notably, the protein coding sequence of each of these genes suggests that they may function in signal transduction pathways essential for myoblast fusion. SNS is a Drosophila member of the immunoglobulin superfamily (IgSF) that is specifically expressed in the fusion competent myoblasts and, by analogy to IgSF members involved in axon guidance, may act as a receptor for signals on the surface of founder myoblasts to initiate fusion. mbc encodes the Drosophila member of a family of proteins that includes C. elegans Ced-5 and human DOCK18O. Recent studies have established that DOCK18O, Ced-5 and MBC are part of a conserved signal transduction pathway that regulates activation of the small GTPase rac 1. Proposed studies include a detailed examination of the role of the cytodomain and ectodomain of SNS, using aggregation assays in cultured cells, myoblast migration and fusion in embryos, and biochemical interaction assays with candidate partners. Identification of other molecules in the SNS pathway will utilize biochemical and genetic interaction assays to evaluate candidate molecules, most notably mbc and Drac 1, as well as novel components that may act in conjunction with SNS. Finally, long terms goals include the isolation of vertebrate homologues of SNS.